[media] DocBook media: fix wrong spacing

There shouldn't be any spaces after <constant> or before </constant>.
This leads to ugly results like: 'image size set by VIDIOC_S_FMT .'

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>